1. Field of the Invention
The present invention relates generally to printing heads for dot matrix printers and, more particularly, it relates to electromagnet designs for use therein.
In dot matrix printers, printing is accomplished by driving selected printing wires in an array of printing wires against a printing surface, typically an inked ribbon adjacent a paper-bearing platen. The individual printing wires are energized by means of solenoids or electromagnets, and during each printing stroke a spring element is tensioned, which pulls the printing wire back to its rest position at the completion thereof. An array of printing wires may produce a complete printed character with each energization of selected members thereof, in which case a 7.times.5 array of 35 wires is typical. Alternatively, the array may comprise a single vertical row of 6-9 wires, in which case successive energization of selected members of the array are required to complete a character. In all cases, of course, the printing head moves so that characters are printed in proper sequence, in seriatim. The head may index from one position to the next in the manner of a typewriter carriage, but in modern matrix printers it is more common for the printing head to move continuously in a horizontal plane, printing "on the fly".
The printing head is a housing having an aperture adjacent the printing face where the printing wires are arranged in the desired array. Within the housing, means are provided to guide the wires, from the respective solenoids or electromagnets to the aperture. The housing also serves as a mount for these driving elements.
Because the aperture is small, being the size of a printed character or a fraction thereof, and the driving elements are relatively large, printing heads tend to be cone-shaped, with the drive elements in the base of the cone at an angle to the printing axis. Thus, the printing wires must curve through that angle to arrive at the aperture and printing face in the printing axis. Since this means that the wires will be rubbing against the guiding means, generating heat and wear and reducing power delivered as printing stroke, it is desireable to keep this angle as small as possible. Further, in prior printing heads there has been an inherent change in the wire angle as the stroke proceeds, which can set up undesired oscillations and introduce axial stresses.
The drive elements always include a core for a winding or coil, with pole pieces for confining and concentrating the magnetic field, and an armature that moves in response to the magnetic field, thus driving the print wire.
Of the many trade-offs facing the designer of a matrix printer, those involving speed and power are the most vexing. Higher speeds are possible with heads of lower mass, but a 7 .times. 1 head must print five times to form a character, as opposed to once on a 7 .times. 5 head. As speed increases, recoil times, pulse shape and many other factors become more critical. A high-powered printing stroke is desired, for example, when several copies are to be printed at once. To do this, larger coils may be provided in the drive elements, but these tend to produce more heat, and they may develop cross-talk problems because of the stronger magnetic fields involved. Thus, spacing between drive elements becomes more critical, with a consequent effect on the mass of the head.
It is the natural tendency of any piece of machinery, in operation, to tear itself to pieces, starting with the weakest part, and this is particularly true of matrix printing heads, where two of the generally desired goals are high speed and low mass. The anchoring of a print wire into the armature of the driving element is one place where failures have been frequent. The printing wire may be a tungsten alloy or other very hard material, and the armature must be a magnetic material. Such materials are difficult to weld under any circumstances, much less to each other. A similar problem exists in attaching the spring means to the armature, where that is necessary (i.e. where the stored spring energy is tensional rather than compressive).
Long before a printing head self-destructs, of course, there will be wear, and any parts that can become improperly aligned will do so. This requires frequent maintenance or, in its absence, more extended downtime for repairs.
2. Prior Art
Understanding of the present invention will be facilitated by considering some recent prior art patents in the field. Where these patents have features in common with the present invention this is pointed out.
In U.S. Pat. No. 3,782,520 and No. 3,833,105 a 7 .times. 1 solenoid-driven printing head is disclosed, and it is one that has achieved significant commercial success. The armatures and printing wires are coaxial, which provides a powerful stroke. This feature is also included in the present invention. The coils are also coaxial with the wires and the armatures, however, and their geometry and structure require that the space between the solenoids be large, with the result that the angle between the printing axis and the armature axis is not constant, and head geometry is considerably enlarged. U.S. Pat. No. 3,802,543 discloses a jeweled nose-bearing aperture for the print wires of the previously-noted patents, and which would be incorporated with equal effect in the present invention.
In U.S. Pat. No. 3,904,011 and No. 3,836,880 there is disclosed a 7 .times. 5 printing head and an electromagnet therefor, where the angle between the printing axis and the printing wires is minimized, but it is, again, not constant, with the same resulting variables in wire path. The patents disclose arrays of electrogmagnets radially mounted in two planes of 18 and 17 respectively, with extensions on the armatures extending toward and very close to the printing axis, the printing wires being attached to the tips of these extensions at a 90.degree. angle. Because of this mounting, some power is lost in transmission to the printing face because of angular change and resulting distortion during the stroke.
U.S. Pat. No. 3,335,659 discloses a drive element for a hammer or character printer rather than a matrix printer, but which is nevertheless of interest. In a printer of this type the electromagnet drives a single hammer once to produce each character. The characters may be on a revolving type drum or on a passing chain. In either case, the hammer presses the character and the paper together (with an inked ribbon or whatever in-between) and a character is produced. Design considerations of a drive element for this kind of printer are entirely distinct from a matrix printer, of course, since only one is required. Power is necessary and speed is desired. To this end, the patentees disclose two U-shaped cores, each with a pair of coils, and four inclined pole faces. The armature has four mating pole faces and is on a flexure mounting on the side opposite the pole faces. The hammer is secured to the armature. This structure is said to be capable of both power and high speed. It is of interest to the present invention because of the U-shaped core, the inclined pole faces and flexure mounting of the electromagnet.
A drive element of minimal mass and less than 0.2 in. cross-section is disclosed in U.S. Pat. No. 3,745,497, the thin construction being adapted for a line printer. A high speed hammer with windings on the armature as well as the core is disclosed in U.S. Pat. No. 3,711,804.
The prior art is significantly vague or even silent on structure adapted to insure easy maintenance and long life; the skilled practitioner can study the above-referenced literature and readily determine potential problem areas; as noted hereinabove, the joinder of dissimilar materials is the frequent locus of life-limiting disorders.